This invention relates to a light emitting device, in particular, having an excellent emission property and a high reliability.
Light emitting devices combining LEDs (light emitting diodes) or other semiconductor light emitting elements and fluorescent elements have been remarked as inexpensive, long-lived light emitting devices, and are widely used as various kinds of indicators, light sources, flat-type display devices, backlight of liquid crystal displays, and so forth.
As typical light emitting devices, there are those mounting semiconductors light emitting elements in resin stems.
FIGS. 14A and 14B show such a typical conventional light emitting device. FIG. 14A is a plan view showing a configuration of the substantially part thereof, and FIG. 14B is a cross-sectional view thereof.
The light emitting device shown here is of a so-called “surface mounting” type, including a package (resin stem) 800, semiconductor light emitting element 802 and sealing element 804 of a resin.
The resin stem 800 has a structure molding a pair of leads 805, 806 shaped from lead frames with a resin portion 803 of a thermoplastic resin. The resin portion 803 has an opening 801, and the semiconductor light emitting element 802 is place therein. Then the semiconductor light emitting element 802 is sealed with an epoxy resin 804.
The semiconductor light emitting element 802 is mounted on the lead 806. An electrode (not shown) of the semiconductor light emitting element 802 and the lead 805 are connected to each other by a wire 809. When en electric power is supplied to the semiconductor light emitting element 802 through those two leads 805, 806, the semiconductor light emitting element 802 emits light, and the light is extracted from an emission surface 812 via the epoxy resin 804.
It is often required to include two or more chips to be mounted in the opening 801 in the semiconductor devices of a type as shown in FIGS. 14A and 14B.
For example, those having two or more semiconductor elements common in emission wavelength, for example, are enhanced in output.
Those having two or more semiconductor elements different in emission wavelength can provide mixed color, thereby to diversify the color representation. In this case, two complementary colors can produce white light.
It is sometimes desirable to mount an element for protecting the light emitting element in a common package. Incase of a light emitting element of a nitride semiconductor, it is often desirable to connect a Zener diode in a parallel opposite directions for the purpose of protecting the light emitting element from static electricity.
However, the light emitting device shown in FIGS. 14A and 14B cannot provide a sufficient space for mounting the chip and for bonding the wire as well. If two chips are packed in the narrow opening by force, the optical axis of the light emitting element will largely offset from the center of the opening, and the intensity profile of the emitted light, i.e., luminous intensity property, will become asymmetrical. Then, the light emitting device cannot provide a uniform emission pattern required in applications such as the back light of a liquid crystal display.
FIG. 15 is a schematic diagram showing a plan-viewed configuration of a light emitting device prepared by the Inventor for trial toward the present invention.
The light emitting device shown here has an approximately rectangular opening 901 formed in a resin portion 903, and chips 902A, 902B mounted on opposed leads 905, 906, respectively, at the bottom of the opening 901. Wires 909A, 909B extending from the chips 902A, 902B are connected to the opposed leads 906, 905, respectively.
As a result of evaluation of this light emitting device, the following problems were found.
The first problem is that a part of an adhesive extruding out upon mounting the chips 902A, 902B causes insufficient bonding of the wires 909A, 909B. For mounting the chips 902A, 902B to the leads, pastes such as silver paste or solders such as gold-tin (AuSn) or gold-germanium (AuGe) solder is usually used.
However, such an adhesive often extrudes on the leads 905, 906 upon mounting. If the extruded adhesive reaches the wire bonding region, it makes it difficult to bond wires 909A, 909B by thermo compression bonding or ultrasonic welding. For example, when a silver paste exists, so-called “breeding” occurs, and it makes wire bonding difficult. Even if they are once bonded, their bonding force will soon degrade significantly.
An attempt of locating the wire bonding site remote from the chip for the purpose of preventing that problem will need a larger opening 901 against the restriction on size.
The second problem lies in that the illustrated rectangular shape of the opening 901 causes side walls of the resin portion 903 to be uniformly thin, and makes the mechanical strength insufficient. This problem becomes serious especially when a soft resin is used as the sealing element buried in the opening. For example, a silicone resin used as the sealing element is advantageous for reducing the residual stress and thereby reducing cracks of the sealing element and breakage of the wire. However, in case the side wall of the resin portion 903 is thin, the relatively soft silicone resin often fails to prevent an external lateral force to act on the chip and the wire. For example, upon picking up the light emitting device by grasping from its side surfaces for assembly and a test, the force actually acted upon the chip and the wire, and often deformed the wire.
The third problem is that the illustrated rectangular shape of the opening 901 need a larger quantity of resin buried therein, and sometimes increases the resin stress. The resin filled in the opening 901 produces a stress upon curing, or thereafter upon an increase of decrease of the temperature.
The degree of the stress depends on the buried quantity of the resin, and tends to increase as the buried quantity increases.
Therefore, the sealing resin filled in the illustrated rectangular opening 901 produced a large stress, and is liable to cause exfoliation of the chips 902A, 902B, and deformation or breakage of the wires 909A, 909B.
That is, the attempt of mounting two or more chips in the light emitting device invites various problems contravening the requirements about the external dimensions.
As reviewed above, conventional light emitting devices were not suitable for mounting a plurality of chips, and had room for improvement from the viewpoint of reliability as well.